1. Field of the Invention
The present invention pertains to flame retardant shipping pallets of polyolefin plastic.
2. Background Art
In the past, shipping pallets were made largely of wood. More recently, numerous materials have at least partially superseded wood-based pallets. For example, pallets of injection molded polymers are being used increasingly. Such polymer pallets have numerous advantages. For example, polymer pallets are capable of being molded in complex shapes which facilitate the shipping of numerous types of articles. Polymer pallets are also easy to clean, which encourages their reuse.
Wood pallets are inherently combustible, and are rather easily ignited. While polymer articles are in general somewhat more difficult to ignite, once ignited they also constitute combustible products, and pound for pound have more potential energy than wood articles. In the shipping industry, empty pallets are often stacked together for reuse or for return to the shipper (“idle storage”). When wood pallets are so stacked and ignited, the fire is generally concentrated in an upward direction. However, when polymer pallets burn, in addition to having greater potential energy (combustibility), the flame can also spread downward by dripping. Thus, the combustion of polymer pallets involves more heat and more potential energy, a combustion mechanism not found in wood pallets. Thus, it is desirable to minimize the combustibility and heat release, and in turn lower the flame spread of polymer based pallets. It is further desirable to provide pallets which mimic the behavior of wood pallets during combustion, and which are preferably improved with respect to combustion properties.
One solution which has been proposed is to produce pallets of polymers which are less flammable than pallets of commodity resins such as polyolefins. However, such speciality polymers, e.g. polyphenylene oxide polymers, are considerably more expensive than the polyolefin polymers conventionally used to manufacture pallets. Such specialty polymers are also, in general, much more difficult to mold than polyolefins. It has also been proposed to add flame retardant compositions which include relatively large amounts of halogenated organic flame retardants. However, such flame retardants can liberate noxious gases when burning, and in addition, are becoming increasingly regulated. It would therefore be desirable to avoid use of halogenated flame retardants.
A standard test for pallet flammability has been established by Underwriters Laboratories, as UL 2335 “Fire Tests of Storage Pallets,” referred to, for example, in WO 00/20495. In one version of this test, the “Idle Pallet Test,” six stacks of pallets are assembled in a 2×3 array with a 6″ longitudinal flue space longitudinally between arrays in a room with a 30 foot high flat ceiling having 165° F. (74° C.) standard response sprinklers having a design density of 0.60 gpm/ft2. An instrumented steel beam is placed near the ceiling, and the pallets are ignited by hydrocarbon soaked cellulosic bundle positioned in the flue space. The parameters assessed include flame spread, maximum steel beam temperature, and number of sprinklers activated. As can be seen, this test is a rather stringent one.
In a second version of the test, the so-called “Commodity Storage Test,” as illustrated in FIG. 1, a 2×2×2 array 1 of pallets 5, each carrying a Class II commodity carton 2, are placed 25 feet (7.5 m) below a 10 M watt heat release calorimeter 3 and ignited by four igniters in the center flue space, each igniter comprising a 3 inch (12.5 cm) cellulosic bundle soaked with 4 oz. (112 g) heptane in a polyethylene bag. Overhead sprinklers 4 at a height of 10 feet (3 m) are activated electromechanically when the instrumentation indicates that a sprinkler activation temperature of 286° F. (141° C.) has been reached. A series of three tests is made, with water application rates of 0.11, 0.21, and 0.31 gpm/ft2. In each test, four parameters are noted: maximum one minute mean total heat release rate; maximum one minute mean convective heat release rate; effective convective heat release rate, defined as the average convective heat release rate measured over five minutes of the most intense fire; and convective energy, the average convective heat release rate measured over the 10 minutes of most severe burning.
Although numerous flame retardants and combinations thereof are known for use in plastic articles generally, the stringent tests required of pallets render flame retardancy results unpredictable. Numerous flame retardants and combinations have been tested, and while many of these have been found suitable for polyolefin articles other than pallets, their use in pallets has not proven acceptable.
WO 00/20495 discloses pallets prepared from specialty resins such as polyphenylene ether resins, polycarbonate resins, vinyl aromatic graft copolymer resins, and polyetherimide resins further including arylphosphate esters and zinc chalcogenides. In U.S. Pat. No. 4,727,102, “self extinguishing” polyolefins are disclosed containing major amounts of ammonium polyphosphate, tris(2-hydroxyethyl)isocyanurate, and melamine cyanurate. However, the large amounts of additives (40%) severely compromise the properties of products prepared from the polyolefin resin.
It would be desirable to provide a polyolefin composition suitable for use in molding pallets which is injection moldable, exhibits good flame retardance in standard tests, and which is commercially cost effective. However, until now, tests of flame retardant systems for use in polyolefin polymer pallets did not result in satisfactory performance.